Layout and method to improve mixed-mode resistor performance

ABSTRACT

A resistor layout and method of forming the resistor are described which achieves improved resistor characteristics, such as resistor stability and voltage coefficient of resistance. A resistor is formed from a conducting material such as doped silicon or polysilicon. The resistor has a rectangular first resistor element, a second resistor element, a third resistor element, a fourth resistor element, and a fifth resistor element. A layer of protective dielectric is then formed over the first, second, and third resistor elements leaving the fourth and fifth resistor elements exposed. The conducting material in the exposed fourth and fifth resistor elements is then changed to a silicide, such as titanium silicide or cobalt silicide, using a silicidation process. The higher conductivity silicide forms low resistance contacts between the second and fourth resistor elements and between the third and fifth resistor elements. The second and third resistor elements are wider than the first resistor element and provide a low resistance contacts to the first resistor element, which is the main resistor element. This provides low voltage coefficient of resistance thermal process stability for the resistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of patent application Ser. No. 10/037,811, filingdate Jan. 4, 2002, now U.S. Pat. No. 6,732,422, A New Layout And MethodTo Improve Mixed-Mode Resistor Performance, assigned to the sameassignee of the present invention, which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a layout and method to improve linearity andreduce voltage coefficient of resistance for resistors used inmixed-mode analog/digital applications.

2. Description of the Related Art

U.S. Pat. No. 6,103,622 to Huang describes a silicide process formixed-mode analog digital/devices.

U.S. Pat. No. 5,924,011 to Huang describes a method for fabricatingmixed analog/digital devices using a silicide process.

U.S. Pat. No. 6,054,359 to Tsui et al. describes a method forfabricating high sheet resistance polysilicon resistors.

U.S. Pat. No. 5,885,862 to Jao et al. describes a poly-load resistor fora static random access memory, SRAM, cell.

A paper entitled “Characterization of Polysilicon Resistors in Sub-0.25μm CMOS USLI Applications” by Wen-Chau Liu, Member IEEE, Kong-Beng Thei,Hung-Ming Chuang, Kun-Wei Lin, Chin-Chuan Cheng, Yen-Shih Ho, Chi-WenSu, Shyh-Chyi Wong, Chih-Hsien Lin, and Carlos H. Diaz, IEEE ElectronDevice Letters, Vol. 22, No. 7, pages 318–320, July 2001 describescharacterization of polysilicon resistors.

SUMMARY OF THE INVENTION

High performance resistors are important devices in the design ofmixed-mode analog/digital circuits. A number of parameters are of keyimportance for these resistors such as resistor linearity, insensitivityof resistance to thermal processing steps, and voltage coefficient ofresistance (VCR).

It is a principal objective of at least one embodiment of this inventionto provide a method of forming a resistor having good linearity, thermalprocess stability, and low voltage coefficient of resistance (VCR).

It is another principal objective of at least one embodiment of thisinvention to provide a resistor layout for a resistor having goodlinearity, thermal process stability, and low voltage coefficient ofresistance (VCR).

These objectives are achieved by first forming a resistor from a firstconducting material such as doped polysilicon. The resistor has arectangular first resistor element having a width, a length, a firstend, and a second end; a second resistor element having a first edge anda second edge wherein the first edge of the second resistor elementcontacts the entire width of the first end of the first resistorelement; a third resistor element having a first edge and a second edgewherein the first edge of the third resistor element contacts the entirewidth of the second end of the first resistor element; a fourth resistorelement having a contact edge wherein the contact edge of the fourthresistor element contacts the entire the second edge of the secondresistor element; and a fifth resistor element having a contact edgewherein the contact edge of the fifth resistor element contacts theentire the second edge of the third resistor element. A layer ofprotective dielectric is then formed over the first, second, and thirdresistor elements leaving the fourth and fifth resistor elementsexposed.

The first conducting material in the exposed fourth and fifth resistorelements is then changed to a second conducting material, which is asilicide, using a silicidation process. The second conducting materialis a silicide such as titanium silicide. The second conducting materialhas a higher conductivity than the first conducting material. The higherconductivity second conducting material forms low resistance contactsbetween the second and fourth resistor elements and between the thirdand fifth resistor elements. The second and third resistor elements arewider than the first resistor element and provide a low resistancecontacts to the first resistor element, which is the main resistorelement. This provides low voltage coefficient of resistance and goodresistor linearity.

The protective dielectric over the first, second, and third resistorelements prevents the resistor from silicidation during subsequentprocess steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of one embodiment of the resistor of thisinvention before the protective dielectric layer has been formed.

FIG. 2 shows a cross section view of the resistor of FIG. 1 taken alongline 2–2′ of FIG. 1.

FIG. 3 shows a top view of the embodiment of the resistor of thisinvention shown in FIG. 1 after the protective dielectric layer has beenformed.

FIG. 4 shows a cross section view of the resistor of FIG. 3 taken alongline 4–4′ of FIG. 3.

FIG. 5 shows a top view of another embodiment of the resistor of thisinvention before the protective dielectric layer has been formed.

FIG. 6 shows a top view of the embodiment of the resistor of thisinvention shown in FIG. 5 after the protective dielectric layer has beenformed.

FIG. 7 shows a top view of another embodiment of the resistor of thisinvention before the protective dielectric layer has been formed.

FIG. 8 shows a cross section view of the resistor of FIG. 7 taken alongline 8–8′ of FIG. 7.

FIG. 9 shows a top view of the embodiment of the resistor of thisinvention shown in FIG. 8 after the protective dielectric layer has beenformed.

FIG. 10 shows a cross section view of the resistor of FIG. 9 taken alongline 10–10′ of FIG. 9.

FIG. 11 shows a top view of another embodiment of the resistor of thisinvention after the protective dielectric layer has been formed.

FIG. 12 shows resistance as a function of voltage for a P⁺ dopedpolysilicon resistor having the protective dielectric layer of thisinvention and a P⁺ doped polysilicon resistor, having the same dopinglevel, without the protective dielectric layer.

FIG. 13 shows resistance as a function of voltage for an N⁺ dopedpolysilicon resistor having the protective dielectric layer of thisinvention and an N⁺ doped polysilicon resistor, having the same dopinglevel, without the protective dielectric layer.

FIG. 14 shows the voltage coefficient of resistance as a function ofvoltage for a P⁺ doped polysilicon resistor having the protectivedielectric layer of this invention and a P⁺ doped polysilicon resistor,having the same doping level, without the protective dielectric layer.

FIG. 15 shows the voltage coefficient of resistance as a function ofvoltage for an N⁺ doped polysilicon resistor having the protectivedielectric layer of this invention and an N⁺ doped polysilicon resistor,having the same doping level, without the protective dielectric layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer now to the drawings for a detailed description of the preferredembodiments of this invention. FIG. 1 shows a top view of a resistorhaving a first resistor element 120, a second resistor element 130, athird resistor element 170, a fourth resistor element 150, and a fifthresistor element 190. The resistor is formed on a substrate 100, such asa silicon substrate having devices formed therein. FIG. 2 shows a crosssection of the resistor at this stage of fabrication taken along line2–2′ of FIG. 1. The boundaries between the first 120 and second 130resistor elements, the first 120 and third 170 resistor elements, thesecond 130 and fourth 150 resistor elements, and the third 170 and fifth190 resistor elements are shown by dashed lines in FIGS. 1 and 2. Theresistor is formed of a patterned layer of conducting material. Theconducting material can be doped polysilicon doped with either N typeimpurities or P type impurities. As shown in FIG. 1, the first resistorelement 120 is a rectangle having a length 20, a width 22, a first end21, and a second end 23. The polysilicon is deposited, patterned, anddoped using techniques well known to those skilled in the art.

The resistance of the resistor is primarily determined by the resistanceof the first resistor element 120, as will be described in greaterdetail later. The resistance of the first resistor element is determinedby the doping of the polysilicon, which determines the conductivity ofthe polysilicon, the length 20 of the first resistor element 120, andwidth 22 of the first resistor element 120.

As shown in FIGS. 3 and 4, a layer of protective dielectric 140 isdeposited and patterned to cover the first 120, second 130, and third170 resistor elements. The fourth 150 and fifth 190 resistor elementsare not covered by the protective dielectric 140. The protectivedielectric can be an oxide, such as silicon oxide, or silicon nitridedeposited and patterned using techniques well known to those skilled inthe art.

Next a silicidation process, well known to those skilled in the art, iscarried out which converts the conducting material in the fourth 150 andfifth 190 resistor elements to a silicide. In this example theconducting material of polysilicon in the fourth 150 and fifth 190resistor elements is converted to a silicide such as titanium silicide,cobalt silicide, or the like. As those skilled in the art will readilyrecognize the silicidation process is usually part of the process forforming contacts in other regions of the substrate 100. The protectivedielectric 140 protects the first 120, second 130, and third 170resistor elements from the silicidation process so that the firstconducting material remains unchanged and the conductivity of theconducting material forming the first 120, second 130, and third 170resistor elements remains unchanged. The protective dielectric 140 alsoprotects the conducting material forming the first 120, second 130, andthird 170 resistor elements from subsequent process steps so that theconductivity of the conducting material in these regions is not changed.Contacts 24 to the resistor can be formed in the fourth 150 and fifth190 resistor elements using methods well known to those skilled in theart.

The conductivity of the silicide in the fourth 150 and fifth 190resistor elements is substantially greater than the conductivity of theconducting material in the first 120, second 130, and third 170 resistorelements. The resistance of the interface 18 between the second resistorelement 130 and the interface 16 between the third 170 and fifth 190resistor elements is low compared to the resistance of the firstresistor element 120 because the conducting material forming the fourth150 and fifth 190 resistor elements has been converted to a silicide.The second 130 and third 170 resistor elements are designed to be widerelative to the width 22 of the first 120 resistor element so theirresistance will be small compared to the first 120 resistor element.

The resistance, R, of the resistor can be expressed as R=R₁+2 R₂+2 R₃+2R₄+R₅. In this equation R₁ is the resistance of the first 120 resistorelement, R₂ is the resistance of the contacts 24 to the fourth 150 andfifth 190 resistor elements, R₃ is the resistance of the fourth 150 andfifth 190 resistor elements, R₄ is the resistance of interfaces, 18 and16, between the second 130 and fourth 150 resistor elements and betweenthe third 170 and fifth 190 resistor elements, and R₅ is the resistanceof the second 130 and third 170 resistor elements. Of these resistancesR₂, R₃, R₄, and R₅ are all quite small with respect to R₁, and theresistance, R, of the resistor is very nearly equal to R₁. This makes itpossible to accurately adjust the resistance of the resistor bycontrolling the doping of the polysilicon, the length 20 of the firstresistor element 120, and the width 22 of the first resistor element120.

Another embodiment of the resistor layout of this invention is shown inFIGS. 5 and 6. As shown in FIGS. 5 and 6 dummy resistor elements 26 canbe formed on either side of the first resistor element 120. The dummyresistor elements 26 can be used to compensate for proximity effectswhen the dimensions of the first resistor element 120 are very small.FIG. 5 shows the resistor before the protective dielectric layer 140 isformed. FIG. 6 shows the resistor after the protective dielectric layer140 is formed.

FIGS. 7–11 show another embodiment of the resistor layout and method ofthis invention. FIG. 7 shows the top view of a resistor and FIG. 8 across section taken along line 8–8′ of FIG. 7. As in the precedingembodiments, the resistor has a first resistor element 32, a secondresistor element 33, a third resistor element 37, a fourth resistorelement 35, and a fifth resistor element 39. In this embodiment, as canbe seen in FIG. 8, the resistor is formed within the substrate 30 and atthe top surface of the substrate. In this embodiment the first 32,second 33, third 37, fourth 35, and fifth 39 resistor elements can beformed by a patterned deposition of impurities in a silicon substrate 30using techniques well known to those skilled in the art. In thisembodiment the first 32, second 33, third 37, fourth 35, and fifth 39resistor elements can be formed by deposition of N or P type impuritiesin a silicon substrate 30.

As shown in FIGS. 9 and 10, a layer of protective dielectric 34 isdeposited and patterned to cover the first 32, second 33, and third 37resistor elements. The fourth 35 and fifth 39 resistor elements are notcovered by the protective dielectric 34. The protective dielectric canbe an oxide such as silicon oxide deposited and patterned usingtechniques well known to those skilled in the art.

Next a silicidation process, well known to those skilled in the art, iscarried out which converts the conducting material in the fourth 35 andfifth 39 resistor elements to a silicide. In this example with theconducting material of silicon the conducting material in the fourth 35and fifth 39 resistor elements can be converted to a silicide such astitanium silicide, cobalt silicide, or the like. As those skilled in theart will readily recognize the silicidation process is usually part ofthe process for forming contacts in other regions of the substrate 30.The protective dielectric 34 protects the first 32, second 33, and third37 resistor elements from the silicidation process so that the firstconducting material remains unchanged and the conductivity of theconducting material forming the first 32, second 33, and third 37resistor elements remains unchanged. The protective dielectric 34 alsoprotects the conducting material forming the first 32, second 33, andthird 37 resistor elements from subsequent process steps so that theconductivity of the conducting material in these regions is not changed.Contacts 34 to the resistor can be formed in the fourth 35 and fifth 39resistor elements using methods well known to those skilled in the art.

The conductivity of the silicide in the fourth 35 and fifth 39 resistorelements is substantially greater than the conductivity of theconducting material in the first 32, second 33, and third 37 resistorelements. The resistance of the interface 38 between the second resistorelement 33 and the interface 36 between the third 37 and fifth 39resistor elements is low compared to the resistance of the firstresistor element 32 because the conducting material forming the fourth35 and fifth 39 resistor elements has been converted to a silicide. Thesecond 33 and third 37 resistor elements are designed to be widerelative to the width 42 of the first 32 resistor element so theirresistance will be small compared to the first 32 resistor element.

The resistance, R, of the resistor can be expressed as R=R₁+2 R₂+2 R₃+2R₄+R₅. In this equation R₁ is the resistance of the first 32 resistorelement, R₂ is the resistance of the contacts 44 to the fourth 35 andfifth 39 resistor elements, R₃ is the resistance of the fourth 35 andfifth 39 resistor elements, R₄ is the resistance of interfaces, 38 and36, between the second 33 and fourth 35 resistor elements and betweenthe third 37 and fifth 39 resistor elements, and R₅ is the resistance ofthe second 33 and third 37 resistor elements. Of these resistances R₂,R₃, R₄, and R₅ are all quite small with respect to R₁, and theresistance, R, of the resistor is very nearly equal to R₁. This makes itpossible to accurately adjust the resistance of the resistor bycontrolling the doping of the silicon, the length 40 of the firstresistor element 32, and the width 42 of the first resistor element. Inaddition to providing the ability to accurately design the resistance ofthe resistor, the protective dielectric keeps the resistance stablethroughout subsequent processing. The design and methods of thisinvention provides a resistor having a low voltage coefficient ofresistance (VCR).

Another embodiment of the resistor layout of this invention is shown inFIG. 11. As shown in FIG. 11 dummy resistor elements 46 can be formed oneither side of the first resistor element 32. The dummy resistorelements 46 can be used to compensate for proximity effects when thedimensions of the first resistor element 32 are very small. FIG. 6 showsthe resistor with dummy resistor elements 46 after the protectivedielectric layer 34 has been formed.

The improvement of resistor characteristics due to the protectivedielectric layer of this invention is shown in FIGS. 12–15. FIG. 12shows a first curve 70 and a second curve 72. The first curve 70 showsresistance as a function of voltage for a P+ doped polysilicon resistorhaving the protective dielectric layer of this invention. The secondcurve 72 shows resistance as a function of voltage for a P+ dopedpolysilicon resistor having the same doping level but without theprotective dielectric layer.

FIG. 13 shows a third curve 71 and a fourth curve 73. The third curve 71shows resistance as a function of voltage for an N⁺ doped polysiliconresistor having the protective dielectric layer of this invention. Thefourth curve 73 shows resistance as a function of voltage for an N⁺doped polysilicon resistor having the same doping level but without theprotective dielectric layer.

FIG. 14 shows a fifth curve 74 and a sixth curve 76. The fifth curve 74shows the voltage coefficient of resistance as a function of voltage fora P⁺ doped polysilicon resistor having the protective dielectric layerof this invention. The sixth curve 76 shows the voltage coefficient ofresistance as a function of voltage for a P⁺ doped polysilicon resistorhaving the same doping level but without the protective dielectriclayer.

FIG. 15 shows a seventh curve 75 and a eighth curve 77. The seventhcurve 75 shows the voltage coefficient of resistance as a function ofvoltage for an N⁺ doped polysilicon resistor having the protectivedielectric layer of this invention. The eighth curve 77 shows thevoltage coefficient of resistance as a function of voltage for an N⁺doped polysilicon resistor having the same doping level but without theprotective dielectric layer.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A resistor, comprising: a substrate; a rectangular first resistorelement having a width, a length, a first end, and a second end formedof a first conducting material on or within said substrate; a secondresistor element having a first edge and a second edge formed of saidfirst conducting material on said substrate wherein said first edge ofsaid second resistor element is wider than said first end of said firstresistor and said first edge of said second resistor element contactsthe entire width of said first end of said first resistor element; athird resistor element having a first edge and a second edge formed ofsaid first conducting material on said substrate wherein said first edgeof said third resistor element contacts the entire width of said secondend of said first resistor element; a protective dielectric formed oversaid first, second, and third resistor elements; a fourth resistorelement having a contact edge formed of a second conducting material,which is different from the first conducting material on said substratewherein said contact edge of said fourth resistor element contacts theentire said second edge of said second resistor element and said secondconducting material has a higher conductivity than said first conductingmaterial; a fifth resistor element having a contact edge formed of asecond conducting material on said substrate wherein said contact edgeof said fifth resistor element contacts the entire said second edge ofsaid third resistor element; and dummy resistor elements adjacent tosaid rectangular first resistor element wherein said dummy resistorelements do not contact said first, second, third, fourth, or fifthresistor elements.
 2. The resistor of claim 1 wherein said firstconducting material is doped polysilicon.
 3. The resistor of claim 1wherein said first conducting material is polysilicon doped with eitherN type or P type impurities.
 4. The resistor of claim 1 wherein saidsecond conducting material is selected from the group consisting ofsilicide, titanium silicide, and cobalt silicide.
 5. The resistor ofclaim 1 wherein said protective layer is silicon oxide.
 6. The resistorof claim 1 wherein said protective layer is silicon nitride.
 7. Theresistor of claim 1, wherein the resistor has two dummy resistorelements respectively adjacent to two opposite sides of said rectangularfirst resistor element.
 8. The resistor of claim 7, wherein each of thedummy resistors is rectangular.
 9. The resistor of claim 7, wherein eachof the dummy resistors has substantially the same distance away from therectangular first resistor element.
 10. A resistor, comprising: asubstrate; a rectangular first resistor element formed of doped siliconhaving an end on or within said substrate; a T-shaped second resistorelement formed of doped silicon having a first edge and a second edgewider than the first edge, first edge contacting the end of the firstresistor element, wherein the first edge is wider than the end; and arectangular third resistor element with a third edge having a widthsubstantially equal to the second edge, comprising a metallic silicon.11. The resistor of claim 10, further comprising two dummy resistorelements respectively adjacent to two opposite sides of said rectangularfirst resistor element, wherein said dummy resistor elements do notcontact said first, second, or third resistor elements.
 12. The resistorof claim 11, wherein each of the dummy resistors is rectangular.
 13. Theresistor of claim 11, wherein each of the dummy resistors hassubstantially the same distance away from the rectangular first resistorelement.